1. Field of the Invention
The present invention relates to a battery pack, and more particularly, to a case structure of a battery pack for sealing a battery assembly.
2. Description of the Related Art
Unlike a primary battery, a secondary battery may be recharged, and may be made into a battery pack and used as a power source for various portable electronic devices, such as cellular phones, laptop computers, and camcorders.
Among secondary batteries, the lithium secondary battery is more suitable for portable electronic devices since it has high driving voltage and high energy density per unit weight. The lithium polymer battery, which is a lithium battery which uses a polymer electrolyte, has the advantage of making portable electronic devices slim and lightweight since it is flexible so as to take on various shapes, and since it is light in weight.
A typical battery pack is basically formed with a unit battery cell which includes an electrode assembly in which a positive plate and a negative plate are wound with a separating plate, and a case or can (hereinafter “the inner case”) which seals the electrode assembly while exposing a positive terminal and a negative terminal. While the unit battery cell is equipped with safety devices, such as a PCM (protecting circuit module), it is assembled into another case (hereinafter “the outer case”) so as to be sealed, thereby providing a battery pack.
When a battery pack with the above structure is applied to portable electronic devices, although it is important to provide high efficiency characteristics of the battery itself as a power source, the battery pack should also provide beneficial mechanical aspects, i.e., safety and strength, so as to protect the unit battery cell.
In addition, in order to keep the portable electronic devices slim, slimness of the battery itself is required. To do this, the structure of the battery pack should be such that the outer case structure forming the shape of the pack is a key factor with respect to slimness of the battery pack.
As far as is known, there are two types of outer cases for a battery pack: a label type and an assembly type. In the label type outer case, the inner case is wound with a film type insulation tape; in the assembly type outer case, the outer case is detachable and is made of a material of substantial hardness, such as plastic and steel, and the case receives the unit battery cells inside thereof in order to seal the case. In the latter respect, if the case is made of steel, an insulating tape is adhered to the outward surface of the outer case.
When comparing the above two types in terms of safety, strength and slimness of the battery pack, the label type has the advantage of slimness, but if the unit battery cell is formed in a pouch shape, it has disadvantages in the areas of safety and strength since it is difficult to obtain a predetermined shape of the unit battery cell.
The assembly type has the advantages of safety and strength of the battery pack since its outer case is made of a hard material, but it has a limitation with respect to slimness. The reason is that it is difficult to decrease the thickness of the plastic outer case, which is manufactured by typical molding methods, below a certain thickness, for example 0.4 mm. The steel outer case has the same difficulty with regard to thickness.
In addition to the difficulty in achieving slimness, the assembly type outer case made of steel causes an inconvenience in use due to the increase in weight of the battery pack. Moreover, since such an outer case has a high risk of short circuit when the unit battery cell is placed in it, its circuit structure is unstable. Furthermore, since the assembly type outer case needs an additional step of binding insulation tape as described above, its manufacturing process is complicated.